(adapted from the applicant's abstract): The long-term objective of this work is to train the applicant to conduct research that will elucidate the mechanisms of the neural control of behavior. In particular, the applicant will further characterize the role of mu opioid receptors (mu receptors) in modulating maternal behavior and investigate a possible mechanism of opioid action by determining percent co-localization of mu receptors and Fos B-positive neurons in the medial preobtic area. Previous research indicates that Pos B, the product of immediate early gene fos B, is necessary for expression of maternal behavior, while activation of mu- receptors suppresses maternal behavior. Nulliparous female rats exhibit maternal behavior toward pups once they have been exposed to them for several days, but there is much variation in the latency to begin maternal behavior. This variability offers a way to investigate the effects of neurochemicals on the timing of maternal behavior expression. The applicant proposes to determine whether: 1) blocking mu-receptor production will affect latency to express maternal behavior; 2) latency to begin expressing maternal behavior is correlated with mu-receptor densities; 3) Fos B and mu receptors are co-locolized. Antisense oligodeoxynucleotide treatment against production of mu receptors and maternal behavior testing will help to elucidate the role of the opioidergic system in suppression of maternal behavior. In situ hybridization and immunohistochemical methods will be used to precisely localize neurons that produce mu receptors and Fos B in adult female rats. The results of these studies will increase our understanding of the neural regulation of maternal care in mammals.